Why are simple shapes like spheres built using polygons?
Creating an "equation" for rendering non-simple objects, such as a tank model, for instance, is much more difficult, especially when it comes to ray-tracing and shading. While for instance a sphere from an equation would be very simple, a sphere next to a complex object would be difficult, and its better to build complicated meshes out of polygons. The reason you can't have math-based meshes next to polygon-based meshes together is because they use different ways of rendering, as well as to keep everything consistent. This is my understanding of it, anyways, and people can correct me if I am mistaken.